In modern third generation computed tomography (CT) scanners, an X-ray source and detector array are secured on opposite sides of the central opening of an annular disk. The disk is mounted to a gantry support for rotation about a subject or object (positioned in the opening) to be scanned. During a scan, the source and detectors image the object disposed within the machine at incremental scan angles. In fourth generation CT scanners the detectors are fixed relative to the object or subject being scanned, and only the source is mounted on the rotating disk for rotation about the subject or object. In both types of systems a process referred to as reconstruction generates a series of two-dimensional images or slices of the object from the captured data.
For "fixed z-axis" scans (the "z-axis" being the axis of rotation of the disk), the disk and its components rotate about a stationary object or subject with the disk fixed at a specific Z-axis location. For "helical" scans, translational movement along the Z-axis is simultaneous provided between the object or subject and the rotating disk. In both fixed and translational scanning systems, precision in the angular velocity, or rotation rate, of the gantry disk is essential for minimization of reconstruction errors.
Timing belts, or cog belts, have been employed in the past to effect a high degree of precision in rotation rate. A standard timing belt is driven by a motor mounted to the stationary frame. Periodic lateral grooves transverse to the major axis of the belt mesh with teeth on a drive sprocket at the motor and a large driven sprocket mounted to the gantry disk. The driven sprocket must be large enough to avoid interference with the central aperture of the gantry and thus allow room for a object to pass therethrough. For this reason, extraordinarily-large timing belts are required in these systems.
A typical prior art scanner requires at least a six meter timing belt. Timing belts of such a large magnitude are very expensive, as they are difficult to manufacture and often must be custom built, and/or purchased in large quantities. Furthermore, the large driven sprockets are specialized and are therefore expensive, available at a cost of $4,000 to $6,000, depending on the diameter. Alignment between the drive sprocket and driven sprocket must be accurate to a high degree of precision, to avoid lateral walking of the belt relative to the sprockets. Timing belts tend to wear rapidly, and therefore must be replaced frequently, for example once per year for a medical scanner. Replacement is an involved procedure, requiring removal of the scanner system from operation for an extended period of time; perhaps a couple of days. This is due to the fact that in prior art configurations, the driven sprocket is positioned between the annular gantry and the fixed frame. Access to the timing belt for its removal and replacement therefore requires complete removal of the gantry from the frame. Positioning of the sprocket on the component side of the gantry is impractical, since the timing belt would interfere with the rotating gantry components.
A further disadvantage of timing belts in CT systems is their tendency to modulate the rotational speed of the gantry at the frequency of their teeth or cogs. The modulation causes artifacts in the resulting images which must be resolved or otherwise corrected by the image processing system.
In addition, mounting the disk for rotational movement requires some type of reliable support so that the disk reliably rotates with little or no lateral movement in the plane of rotation. In the typical prior art system, standard bearing arrangements, with highly machined races and balls, are expensive. Because of the weight and size of the disk the bearings tend to wear, and are difficult to replace. One solution to this problem has been to mount the disk for centerless rotation on rollers such as shown in U.S. Pat. No. 5,473,657 issued Dec. 5, 1995 in the name of Gilbert W. McKenna, and assigned to the present assignee.